The present invention relates to a process for the regeneration of used deNOx or dedioxin catalysts. Catalysts of this type are used in so-called deNOx or dedioxin installations for reducing and breaking down nitrogen oxides and/or in particular halogenated dioxins and furans in flue gases or other exhaust and off-gases.
The process known as selective catalytic reduction, or SCR for short, is one of the possible options for lowering or even substantially lowering the levels of nitrogen oxides NOx, i.e. a mixture of NO and NO2, formed for example during the combustion of fossil fuels in combustion plants. In the SCR process, the nitrogen oxides are converted into nitrogen and water using ammonia or substances which form ammonia under the system conditions as reducing agent and using a catalyst. Since the catalytic reactions proceed on the surface of the catalyst, a large specific surface area has to be provided through the use of correspondingly porous materials for the reaction. This requirement is met by the use of homogeneous ceramic catalysts, for example in honeycomb form. Most of a catalyst of this nature consists, for example, of the base material titanium dioxide TiO2 in which the active metal compounds, in particular V2O3, WO3, are homogeneously distributed. However, the catalyst may also be applied as a coating to a support, for example a metal sheet.
Under oxidizing conditions, identical or modified catalysts can also be used to lower the levels of emissions of organic products of incomplete combustion in off-gases from combustion plants, such as for example halogenated dioxins and furans. In this context, reference is had to the disclosure in international publication WO 91/04780.
There are transport processes upstream and downstream of the chemical reactions which take place on the catalyst surface. Following adsorption of the reaction partners on the internal surface of the catalyst, chemical combination between the reaction participants and the catalyst leads to a lowering of the activation energy which is absolutely imperative for the reaction to commence. A consequence is that the reaction is accelerated or the equilibrium is established.
If these active centers are blocked, for example by the accumulation of alkali metals and alkaline earth metals or their compounds which are contained in the fly ash, so that the activated NH3 adsorption required is partially impeded, the activity falls. In addition to this deterioration to the active areas of the catalyst surface through adsorbed catalyst toxins, the pores become blocked, for example, by calcium sulfate (CaSO4) and ammonium hydrogen sulfate (NH4HSO4) which are formed. Since the catalyst cannot be 100% selective with respect to a specific reaction, the catalyst also promotes some secondary reactions, including the conversion of SO2 to SO3, in an order of magnitude which is relevant. Although this reaction can be minimized by the composition of the catalyst, the fact remains that the small amount of SO3 is sufficient to react with the unreacted NH3, which is referred to as NH3 slippage, and H2O to form various salts, primarily to form ammonium hydrogen sulfate and ammonium sulfate (NH4)2SO4 or to combine with the fly ash.
These compounds form at temperatures at which condensation takes place when the temperature drops below the dew point of ammonium hydrogen sulfate. They may be deposited on the catalyst and in addition, together with adhesive particles, for example ash, fine dust, SiO2, Al2O3, may block the pores and thus lower the activity of the catalysts.
Therefore, the nature of the composition of the compounds which may be deposited on the catalyst is dependent on the composition of the fly ash, of the flue gas and of the operating temperature. They are generally alkali metal and alkaline earth metal compounds which are contained in the fly ash as oxides and, on account of their reaction with SO3, as sulfates and which are either deposited on the surface together with other compounds contained in the fly ash, such as for example SiO2 and Al2O3, and block the pores, or, on account of their electron donor properties, block the active centers and thus prevent the activated NH3 adsorption required.
The object of the present invention is to provide a method of regenerating a deNOx or deDioxin catalytic converter which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides for a process by means of which the number of the active centers available for the catalysis is increased as far as possible or as desired, namely, for example, up to the activity of the fresh catalyst or even beyond, in order in this way for the catalytic converter, i.e., the catalyst, to be fully or partially regenerated.
With the above and other objects in view there is provided, in accordance with the invention, a process for regenerating a used deNOx or dedioxin catalytic converters, which comprises washing a catalytic converter with a solution of surface-active substances in a liquid with a simultaneous addition of metal compounds creating active centers.
In other words, the above objects are achieved by the fact that the catalysts are washed with a solution of surface-active substances in a liquid, preferably water, with the simultaneous addition of metal compounds which create active centers.
As a result of this measure, deposited contaminants and chemisorbed compounds and ions are removed, old active centers are made available once more and additional active centers are created. In addition, in this way it isxe2x80x94quite surprisinglyxe2x80x94also possible to increase the activity compared to the fresh catalyst. The catalysts which have been treated in this way can be refitted into a deNOx, dedioxin or combined plant with their restored activity.
Washing in, for example, aqueous liquors is a complex operation in which numerous physical and chemical influences interact. This is understood as meaning both the removal of water-soluble deposits by water or by aqueous solutions of active washing substances and the detachment of water-insoluble deposits. In the process, it is possible to prevent redeposition of the insoluble fractions which have already been detached, for example by acoustic irradiation or by stabilizing the dispersed fractions. The water serves as a solvent for washing agents and for soluble compounds and as a transport medium for the dispersed fractions. The washing operation is initiated by the wetting and penetration of the substrate. This can be achieved quickly and completely if the high surface tension of the water is reduced substantially by surfactants as important washing agent components. The physical separation of the deposits from the substrate is based on the nonspecific adsorption of surfactants at various boundary surfaces which are present in the process. Substances with a low solubility are solubilized in molecularly dispersed form by surfactant micelles. The adsorption of washing agent constituents induces changes in the interfacial chemical properties and is consequently a precondition for good detachment.